This invention relates to a bubble ink jet printing system and, more particularly to a printhead having a temperature sensitive material incorporated therein which serves as a temperature sensor to effectively control heat generated during the printing operation.
Bubble jet printing is a drop-on-demand type of ink jet printing which uses thermal energy to produce a vapor bubble in an ink-filled channel that expels a droplet. A thermal energy generator (printhead), is located in the channels near the nozzle a predetermined distance therefrom. A plurality of resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink is ejected from a nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separating of the bulging ink as a droplet. The acceleration of the ink out of the nozzle in which the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
A problem with prior art printhead operation is the increase in temperature experienced by a printhead during an operational mode. With continued operation, the printhead begins to heat up, and the diameter of the ink droplet begins to increase resulting in excessive drop overlap on the recording media thereby degrading image quality. As the printhead experiences a further heat buildup, the ink temperature may rise to a point where air ingestion at the nozzle halts drop formation completely. It has been found that, at about 65.degree. for a typical ink, printhead operation becomes unreliable. There is also a lower temperature limit for reliable operation which varies for different inks and device geometries. This limit might, for example, be about 20.degree. C. for an ink and device designed to function reliably up to, for example, 60.degree. C. At the same time, it is desirable to offer an extended range of ambient operating temperature, such as 5.degree. C. to 35.degree. C., so that it will be necessary to provide for warming up the printhead. It is also desirable to minimize the time required to warm up the printhead, so that first copy (print) out time is acceptable. The printhead characteristics and machine environment requirements have the following impact on the thermal design of the system. The generation of heat during operation (which becomes a greater problem as print speed, duration, and density increase) makes it necessary that the printhead be connected to a heat sink, which is efficient in transferring heat away from the printhead. The efficiency of the heat transfer away from the printhead will be enhanced by the cooler the heat sink is relative to the printhead. Because of the range of ambient temperatures to be encountered (assumed to be 5.degree. C. to 35.degree. C., but not limited to that range), and because of the temperature uniformity requirement, and further because it is less complicated and less expensive to control temperature by heating than by cooling, it is advantageous to set the nominal printhead operating temperature at or near the maximum ambient temperature encountered. Because of the desired minimal first copy (print) out time, as well as the desired efficiency of the heat sink, it is also advantageous to situate a temperature sensor and heater as close as possible (thermally) to the printhead, and as far as possible (thermally) from the heat sink.
Temperature regulation typically is achieved in the prior art by using a combination of a temperature sensor and a heater in a feedback loop tied into the printhead power source. For example, U.S. Pat. No. 4,250,512 to Kattner et al. discloses a heating device for a mosaic recorder comprised of both a heater and a temperature sensor disposed in the immediate vicinity of ink ducts in a recording head. The heater and sensor function to monitor and regulate the temperature of a recording head during operation. Column 3, lines 7-24 describes how a temperature sensor, a thermistor, a heating element, and a resistor operate in unison to maintain the recording head at an optimum operational temperature to maximize printing efficiency. U.S. Pat. No. 4,125,845 to Stevenson, Jr. discloses an ink jet printhead temperature control circuit which uses a heater and a temperature sensing device to maintain a recording head temperature above the preset temperature level. An output from the temperature sensing device drives an electrical heater which regulates the recording head temperature. The temperature sensing device is a resistive element attached to the bottom side of the printhead by thick film techniques. U.S. Pat. No. 4,704,620 to Ichihashi et al. discloses a temperature control system for an ink jet printer wherein the temperature of an ink jet printhead is controlled by a heater and a temperature sensor which collectively regulate heat transfer to maintain an ink jet printhead within an optimum stable discharge temperature range. The temperature control circuit, as shown in FIG. 7 of the patent, utilizes an output from a comparator circuit and control signals from a signal processing circuit to regulate printhead temperature based on the output from the temperature sensor. U.S. Pat. No. 4,791,435 to Smith et al. discloses a thermal ink jet printhead temperature control system which regulates the temperature of a printhead via a temperature sensing device and a heating component. The temperature sensing device, comprised of either a collection of transducers or a single thermistor closely estimates the temperature of the ink jet printhead and compensates for an unacceptable low printhead temperature by either cooling or heating the printhead as needed. U.S. Pat. No. 4,686,544 to Ikeda et al. discloses a temperature control system for "drop-on-demand" ink jet printers wherein a heat generating electrode, positioned between layers of insulating and resistive material of a printhead substrate, controls the temperature of the printhead during operation, Column 4, lines 7-25, describes how an electrothermal transducer delivers the heat required to maintain the ink jet printhead at an optimum temperature level to maximize efficiency printing efficiently. U.S. Pat. No. 4,636,812 to Bakewell, while disclosing a thermal printhead, also teaches using a heater and temperature sensor supported within a laminated layer near the marking resistors.
U.S. Pat. No. 4,738,871 to Watanabe et al. discloses a heat-sensitive recording head which makes use of laser-made holes to control the resistance of the heater resistors. These laser-made holes are also used to control the temperature which is directly related to the resistance. A method for making the laser holes is also provided.
U.S. Pat. No. 4,772,866 to Willens discloses a device including a temperature sensor. The temperature sensor uses the semiconductor material (polysilicon) which is already part of the device.
U.S. Pat. No. 4,449,033 to McClure et al. discloses a thermal printhead temperature sensing and control system. A sensor is made of a thermo-resistive material (Col. 4, lines 23-24) which runs parallel to the printhead leads. Means are provided for the temperature control circuitry for the printhead. The sensor can also sense a temperature change in a single printhead element (Col. 1, line 55). The sensor is situated above the printhead leads and separated from them by glass (FIG. 2, Numbers 10, 11).
The above references disclose various types of discrete temperature sensors which provide sensitivity for the particular system that they are used in. However, more precise temperature sensing and heater control may be required for certain print system depending upon printhead geometry, print speeds, and ambient operating temperature range. An optimum physical arrangement for a heater and sensor is to be in close proximity to the printhead. An optimum material from a manufacturing and economic standpoint is, for the temperature sensor to be formed from the same material as the resistor heating elements in the printhead. This goal, however, has not been achieved because the fabrication tolerances for the resistor are not sufficient for the purposes of forming sufficiently accurate thermometers on a plurality of printheads. In other words, it is heretofore not been possible to fabricate a plurality of printheads which may be required for a specific print system so that each temperature sensor for each printhead would be within a specific and consistent temperature tolerance range. A typical temperature coefficient of resistance of polysilicon is 1.times.10.sup.-3 /.degree.C. and a typical resistance tolerance is .+-.5%. Thus, a thermistor formed near the resistor array would be inaccurate by as much as .+-.50.degree. C. Depending on the temperature control and printhead performance, sensitivity to temperature for a specific system, a thermometer would have to obtain an accuracy of .+-.1.degree.-5.degree. C.
Thus, heretofore, it has not been possible to form a thermistor in close proximity to the printhead and of the same material as the heaters or the printhead. According to the present invention, however, it has been found that the accuracy of a thermistor of the same material as the printhead heater elements can be improved so that its accuracy is within the desired temperature range (of 1.degree.-5.degree. C.) by trimming the thermistor, or, by trimming an external resistor in series with the thermistor while holding the printhead at a desired temperature control set point. More particularly, the present invention is directed towards a thermal ink jet printhead including: a substrate support; an ink heating resistive layer disposed within said substrate comprising individual resistive elements in communication with an adjacent ink filled channel; and a second temperature sensitive resistive layer disposed within said substrate and proximate to said resistive layers, said temperature sensitive layer having an electrical connection to a temperature control circuit.